Allow implicit casts from n-th degree const pointers to nullable const
pointers of degree n+1.  That is:

    fn f() void {
        const s = S {};
        const p = &s;
        g(p);   // Works.
        g(&p);  // So does this.
    }

    fn g(_: ?&const &const S) void {  // Nullable 2nd degree const ptr.
    }

Fixes #731 some more.